Cathode ray tube



May 3, 1960 Filed May 31, 1957 K. scHLEsINGER 2,935,643

CATHODE RAY TUBE 2 Sheets-Sheet 1 Y van-r. 26 iswam May 3, 1960 Filed -May\31, 1957 K. SCHLESINGER CATHODE RAY TUBE 2 Sheets-Sheet 2 INVENTOR.

itcd

- `CATHODE RAY TUBE KurtSchlesinger, La Grange, Ill., assigner to Motorola, Inc., Chicago, Ill., a corporation of illinois Y This invention relates to cathode-ray .image display devices and more particularly tocathode-ray tubeswhich are of reduced depth or thickness so that a comparatively x flat structure is formed.

VMost present day'I electron'image display devices, or fcathode ray tubes, are rather long structures with an electron gun spaced a considerable distance from the screen or target. "This adds to the bulk of any apparatus in which the device is used. Accordingly, effort has been directed to the problem of devising a more compact cath- `ode ray tube. VMy copending application, filed April 4, 195 6, and bearing Serial No. 576,204 describes and claims p a cathode ray tube in which the-electron beam is directed r"into .an electron mirror and thenv folded or `redirected to a position for impingem'ent upon a screen, thereby formking a device which is fiat and compact. Scanning of the "beam in one direction is obtained by defiection parallel to, *4and beyond,' theviewing screen before entry 'of' the beam -into vanelectron-optical refiectingdevice, used tof fold the vplane of deflected rays by almostlSO degrees. -Thiselec- -tron opticaly device whichv canvmake use lof electrostatic v fields or of mixed electric and magnetic fields, the Jdesign of which is part of this invention, is hereafter referred to as an electron mirror. The electron mirror is elongated vand supported inr arfirst planesubstantially` parallel tothe fscreen' to accommodate the beam in. its various deflected positions prior to further deflection in a second plane which is perpendicularto the first plane.

Such an electron mirror for redirecting the beam Amay include both magnetic and electrostatic fields for reversing the path of travel of the beam and elevating it or directing it away from the plane of. 'incidence into the mirror. However, details of the mirror construction have decisive influence on` beam' deflection, V'particularly d'eiiection oc- ,curring before entry into, or during passage through, the mirror. Furthermore, since the beam' is`not ilamentary, Mor infinitely thinsome defocussing or astigmatism'occurs "b'ecause of the fmirror,V and it is; of course, desirable to "minimize' this effect. Obviously, the' reduction of fsuch j''lefocussingl wouldnpreferably beaccomplished in al way which does not vcomplicate beam '-deection circuits used for the tube. v

'An object of lthe present inventionvis Vto provide a"i"ela jtively thin or'flat electronimagedisplay device wherein defocussing of the electron beam is minimized.

l ,Another'obje'ct is to provideacathoderay tube wherein *the beam is uredirected'` or reiiexed to reducethedirnensions ofthe tube kandtheredirecting apparatusis related v,to thel beamdeiiecting'` anglev to simplifyV the deflection circuits required. Y A ,n Still another object is'to"`provide' "a cathode `ray ",tube' 'of rates Patent g r flat.construction which produces a scanned pattern of tron image display device in whichthelfbeamfis'defleeted through various angles and-isthenredirected:byanelec Another feature is Vthelprovision of a4 novel lmagnetic l eiectron mirror wherein thev intensity of theY mirror-mag netic field varies throughout thelength thereof so `that maintained.

Still another featureof lthe invention isgthe provisionof an electron mirror which employs combined magnetic and electrostatic fields for electron reliection' Withfafscreen thereof disposed in skewed relation'with respect'rto-the -mirror so that the angle of beam-arrival thereon, vras'it emerges from difierentfportions of the -mirror, is constant and defocussing effects in the electron-mirror are. minimized. v

Fig. l is a rear elevational view of aV cathodegray tube constructed in accordance'with the invention; Fig. 2 is a sidefview of the tube of Fig. l; Fig. 3 is a sectionalrview along the line 3'-3 of Fig. l; Fig. 4 is a diagram useful in `explaining the invention; fFig. 5 isa diagram useful in explaining a modified form of the invention; and Y Fig. 6 is a view ofthe modified form correspondingzto Vthat'of Pig. 3. l, j

The invention provides a cathode rayyimage display device of shortdepth or thickness.

Y supported in angular relation with respect to one another,

and the intensity of the magnetic fieldy is related to thek s angular position of the screen so that beamfocus-andaf j desirable imagealignment upon the screen' are Vobtained,k Q

as -the-beam is scanned through the scanning plane. g The described relation may be established by variation inthe i' fmagnetic field along the electron mirror so that the diam- Y eter of the beam orbit (or beamtime withinthemirror) f changes `with'fsweep-'angle to provide constant transit time inthe mirrorlfield. It is further possiblegtotilt the]L l j screen with `respect to -theeelectron'mirrorso lthat a conlstantscreen impingementf.fangle-l is `'maintained andithe l image-,is-notaslcew on-the screen.

Considering-the'. invention in greater ;detail,Figs; 1,.- 2 and 3 whichy -show'oneform ofthe invention, wilrnow be described. Y Theimage display-.device-or tubegincludesan .f evacuated enclosure land an electron gun 12. en-

n closure. 101may--beformed -pof glass Withy-conveX-fsidesfto Y withstand-airpressure.l Gun 1,2iszcqnnected tofasuitable beam control'circuit'.(.notsshownlfor a given application of the- 'cathode yray ltube. ln thisfformlof thefinvention screen-'18 is a; coatin'gdeposited uponitheface: of :theen- `:closure ltland connected-tov apositive potential sourcefto form a bea-m target forv the tube. VK'flt should befnotedL at t theenclosureiljcanbe formedof transparentmatenalso frihatfuscreenels 'ma'ylee-vietati?,frornfeiugefE de fflffhe device if this is desirable. l

a uniform focal throw of the beam vfrom thefmirroriisV Thedevice includes a-magnetic electronv mirror into which a b eamisgdirected 'from av gun and across whichthe beam is scanned in2 av Agiven scanning plane by suitable first defiecting means. Y

screenand the pole pieces of the mirror'arefskewed, or f fibre-leerme@ une@ ythese angles generally being equal and on opposite sides of a central beam path 25.

The deflected beam is directed past and parallel to shield 27 into an electron mirrorr28 which is disposed beyond the upper edge of screen 18 (Figs. 1 and 2), so that as the beam emerges from mirror 28 it may be directed into the screen 18. The electron mirror 28 includes a pair of spaced, L-shaped magnetic pole pieces 30 and 31 in the gap between which a magnetic eld is established for collimating the beam, or returning it in a path 25a parallel to its path of entry as shown in Fig. l. An external exciter magnet 33 is shown in linx-coupling relation with pole-pieces 30, 31. The exciter comprises coils 34 which are energized by a direct current to provide the magnetic eld. The same function may also be performed by suitable permanent magnets.

The electron mirror 2S further includes a pair of nonmagnetic plates 36 and 37 disposed in spaced relation between pole pieces 30 and 31 so that they are straddling the scanning plane of the beam. In the gap between these plates there is formed an electrostatic eld by energizing the plates with a voltage of the polarity shown.

Assuming that the beam is travelling the central path 25 it may be seen that upon entering mirror 28 the magnetic field will turn the beam through a given arc and return it along path 25a as shown in Fig. 1. The beam will further be elevated, or directed away from the plane of incidence by virtue of an electrostatic eld set up between the electrostatic plates 36, 37. These are polarized as shown in Fig. 2, with the positive plate on the screen side of the beam scanning plane. Thus, the beam may be elevated by controlled amounts under the influence of suitable scanning voltages from a vertical sweep generator 26. (Plates 38, 39 may also be coupled to circuit 26 for scanning purposes). In Fig. 2 various degrees of vertical deflection are designated paths 25b, 25e and 25d. It is contemplated that the beam hit the screen center after electrostatic elevation by approximately 25.

In Fig. l the edges of mirror 28 (members 30, 31, 36, 37) which face the gun 12, are parabolic in shape with the focal point of the parabola at the center of deflection of plates 20, 21. Under this condition, it can be shown that the beam, after being predellected through angles A and B, will be collimated or returned in paths parallel to path 25. This parabolic mirror shape and the resulting beam collimation also cures the tendency for keystone shaping of a pattern on screen 18 where the pattern is smaller near the mirror (top of the screen) than it is at more remote positions on the screen when the beam is swept in both directions on the screen.

It may be seen in Figs. 1 and 4 that when the beam is deflected through angle B and travels path 42 the angle through which the beam turns in mirror 28 will be pi plus B. Furthermore, when the beam is deected through angle A to travel path 44 the turning angle in mirror 28 will be pi minus A. Accordingly, the transit time in the mirror when the beam is travelling path 42 is considerably longer than it is when the beam is travelling path 44. Therefore, since the beam is within the mirror 28 for a longer time it will emerge with an increased elevation angle (with respect to the scanning plane), namely, along path 42a. Similarly, at path 44 the beam is within the mirror for a shorter time, and the elevation angle will be smaller than at center and the beam will be depressed as shown along path 44a. Under these conditions deection of the beam through angles A and B in its scanning plane will produce a display trace 46 which is skewed with respect to the central axis of the tube. Furthermore, it has been found that this trace does not stay in focus on the plane of the screen 18, the discussion of which is given in connection with Figs. and 6.

In order to correct this deection defocussing and produce a sharp horizontal trace 48, the transit time within mirror 28 is suitably controlled in relation to the point of incidence on the electron mirror. The transit time de- 4 pends upon the radius of curvature within the mirror and the radius is governed by the formula:

naah/E B where B is the magnetic field strength and E is the electron beam potential. The magnetic field strength may thus be varied along the length of mirror 2S to provide the desired transit time therein. As most clearly shown in Fig. 3, this may be done by making the gap width between the magnetic pole pieces 30 and 31 greater at the portion of the mirror in which the beam is turned through less than than it is at the portion of a mirror in which the beam is turned through an angle of greater than 180, i.e. gap 51 is greater than gap 50. The electric eld, on the other hand, is not modied, as is the magnetic field, but it is kept uniform all along the extent of the electron mirror. This is accomplished by keeping the spacing between the non-magnetic pole pieces 36--37 constant. The electric eld set up between these electrodes by voltage (D.C.) is then constant throughout.` As a result, by decreasing the path curvature at one end of the mirror and increasing it at the `other end of the mirror, the electron transmit time is made constant and since the electrostatic field is also constant throughout, the beam leaves the mirror with a uniform angle of elevation. Moreover, since the length of the beam path in the mirror is now constant, the focal throw from the mirror outward is also constant.

In a practical construction of the invention the magnetic pole pieces 30 and 31 may be 12" in length and formed of Mu metal. The scanning angles A and B may be 20 each. The air gap 50 may be 0.67l and the gap 51 may be 0.83. A formula for the ampere-turns providing the magnetic eld for the pole pieces is as follows: ni=4.2 tan M\/E where M is the angle of elevation from the plane of incidence and E is the beam potential. The necessary gap widths between the poles in order to avoid grazing of the mirror by the beam, may be determined from the following formula:

9:32513 tan M where R is the radius of the trajectory within the mirror and M is the angle of elevation from the beam of incidence. Y

As previously mentioned the electron mirror may cause defocussing of the beam such that a properly focussed trace does not lie in a plane parallel to the mirror, e.g., the scanning plane defined in Fig. 1 by paths 25, 42 and 44. I have found that the trace may originally focus along a line which deviates from the scanning plane by as much as 27. However, if the above transit time correction is applied this deviation may be reduced to 9 or less.

This condition is illustrated in Fig. 5 wherein the beam is deected in the scanning plane through angle A to travel path 55 and through angle B to travel path 57. After emerging from the mirror 28, only pole pieces 30, 31 thereof being shown for simplicity, the trace 60 on screen 18 is found to be partly defocussed when the screen is positioned parallel to the plane of mirror 30. Proper focus may be restored, however, by tilting the screen with respect to the scanning plane through an angle S until trace 60a is properly maintained in focus on the tilted screen 18a.

Fig. 6 is a view of the tube corresponding that of Fig. 3 and shows the screen 18a tilted with respect to the scanning plane which coincides with the plane of symmetry of the mirror 28.

This invention provides therefore, an electron image display device which may be constructed as a. comparatively flat unit. Necessary focussing of the beam is maintained throughout the first scanning operation (as the beam is deected through angles A and B) and astigmatism, which could otherwise be caused by mirror sagesse@ 1128, is thus gprevented. The method of correctingf such defocussing'involves comparatively simple structural'aro rrangements within the tube. "This permits'the `use of Ascanning signals of conventional 'wave forms, which may be produced Vby simple sweep circuits atV 23 and'zf' Itzshould also'be pointedy out that the cathode ray tube 1 4asdescribed will permit an unobstructed'view'vof the screen v18 from either one or both sides thereof, which feature may provide utility in certain applications.

"Fur'thermore, the tube will be free from ion-burn since an electron mirror of the electromagnetic type is used `which does not reiiect ions originatingginthe 4electron beam. 4

I claim: Y y 1. An electron beam image display device including in combination, van elongated electron lbeam mirror for rel rdirecting a beam, means supplying an electron beam and directing the same toward said mirror, deecting means for scanning said beam in a given plane and directing 'the same into various portions of `said mirror, saidv mirror including spaced portions and means'to establish j therebetween a magnetic iield for reversing the travel lof saidA beam and an electrostaticiield for directingsaid beam outwardly from said plane, a beam target spaced from said planeso that said beam'may be directed onto the same, said beam target being angularly disposed Ywith respectto said given plane and such angular disposition v being related` to the intensity of said magnetic field along "said mirror so that focussing of said beam uponsaid Y target is obtained throughout a given cycle of beam del" ection 'in saidplane. Y

2. An electron image display device including incomsaid beam away froml said plane, a beam target spaced outwardly from said plane and positioned so that'saidY beam isl directed onto the sarne,V the` plane of said beam targetand Yat least ,one of said pole portions of said Y mirror being" disposed in non-parallel relation with respectto 'one another, andv the lintensity of saidfmagnetic ,"'field along said mirror being Vrelated to the angular posi- "Ttionof said beam targetv so thatfocus of said bamfupon said target is maintained throughout a given cycle of beam deflection in said plane.

3. An electron beam image display device including in combination, an elongatedlelectron beam mirror for redirecting a beam, means supplying an electron beam vand directing the same toward said mirror, deliecting vmeans for scanning said beam in a given plane and directingthe same into various portions of said mirror,

said mirror including spaced portions and means to establish therebetween a time tixed magnetic ield for reversing said beam and a time fixed electrostatic field for directing said beam away from said given plane, a beam .target spaced outwardly from said given plane and terminating on the side of said mirror toward said means supplyingvan electron beam so that said beam may be directed onto the same, means providing a time variable field operative upon said lbeam after the same enters said mirror for deecting said beam in a furtherplane substantially perpendicular to said given plane, the plane of said beam target and at least one of said spaced portions l, of said mirror being skewed with respect to one another,

with the intensity of said time fixed magnetic iield and said time variable electrostatic lield along said mirror vbeing related to the angular position of said beam target so that focussing of said beam upon vsaid target is obsaidgiven plane.

4. An' electronimage @display ldevice including :incom-v fbination, an longatedfelectron beam mirror forredirecting a beam, meansfsupplying an electron beamr and di`v recting thesaine toward'said mirror, deilecting means rfor scanningusaid beam in'a given plane, said mirror including a plurality' ofelectrode niembersspaced onopposite sides ofsa'id 'given' plane,'said electrode 'members being adapted to provide a`magneticfiield across said plane for redirecting saidbeam and an 'electrostatic field across said planeforfdirec'ting said beam outwardly from said plane, la beamtarg'et spaced loutwardly from said planeand positioned sthat'lthe beam isfdirected onto the same, at leastonef'offsaid:electrode members being disposed in non-parallel-"rela'tion withrespect to said given plane sothat the intensity of at least one of the 'e'lds in said 'mirror' is `yari'ed therealong to provide uniform redirecting of the beam by said mirror with respect to said target.

5. An; electron image display device including'in com bination, .an elongated electron bearnj mirror for redirect-r ing a beam, means fsupplying an electron beam and directing the sametowt'rdsaidmirror,` deflecting meansv for scanning said"bea`m fin a given plane, said mirror including a plurality of electrodeV members spaced on op-V posite sidesvof said given plane, said electrode members being adapted to provide a magnetic held-'acrossl said plane for redirecting said beam and an electrostatic field across said plane `for directing said beam outwardly from said plane', `a beamtarget'fspaced outwardly from said plane Vandvpositioned sothat the beam may .be directed onto the same, a pair ofsaid electrode members disposed invariable gap relation with respect to said given plane with the intensity of said magnetic iield across said mirrorl being varied therealong in correspondence with the angle of dee'ction of said beam inl' said-v given plane so that'the .Y

beamt-ransit *time in said. mirror is constantwherebyV focus of said beam on said target is substantiallymaintained in the deflected positions thereof.

6. VAn electron' image vdisplay device including'in combination, an elongated electron ybeam mirror for redireeting the beam,A means for supplying an electron beam and directing the same toward said mirror, deiiectingfmeans for scanning said beam in 'a given plane,L said mirror cluding a plurality of pairs of 'field producing members disposed on opposite sides of said plane, a first pairof said members having a substantially parabolic edge facing said dee'ctingfmeansland being adaptedtoprovide a magnetic ieldi of;substantially parabolic( shape across said plane for reversing and collimating said beam by turning the same through greater than in one'scanned position and less than 180 in another scanned position, a second pair of said members being adapted to provide an electrostatic field across said plane for directing said beam away from said plane, a beam target spaced outwardly from said plane and positioned so that the beamVv` may be directed onto the same, said first pair of mem-Vv bers being spaced more closely wherein said beam is turned through greater than 180 than the same are spaced wherein said beam is turned through less than 180l Y so that the magnetic eld intensity is; varied along said mirror in accordance with the angle through which the beam is turned and substantially constant transit time of said beam in different positions within said mirror is maintained. j

7. An electron image display device including in combination, an elongated electron beam mirror forredirecb ing a beam, means for supplying an electron beam and directing the same toward said mirror, deiiecting meansV for scanning said beam in a given plane,V said mirror including a plurality of iield producing members'disposed n on opposite sides of said plane, a first pair of saidjmembers having a substantially parabolic edge Ifacingsaid rdeflecting means and being adapted to providea magnetic field of substantially parabolic shape across said plane for turning said beam through an arc and returning the same along parallel paths with the arc subtended being variable and dependent upon the scanned position of said beam produced by said deecting means, a second pair of said members being adapted to provide an electrostatic ield across said plane for directing said beam away from said plane, a beam target spaced outwardly from said plane and positioned so that the beam may be directed onto the same, said first pair of members being spaced more closely at positions Where said arc is angularly greater than where the same is less so that the magnetic eld intensity is varied in accordance with the `angle through which the beam is turned, whereby transit time of said beam in said mirror is substantially constant and focussing of said beam upon said target is maintained during beam deflection in said plane.

8. An electron image display device including in combination, an elongated electron beam mirror for reversing the direction of travel of a beam, means supplying an electron beam and directing the same into said mirror, deecting means for scanning said beam in a given plane, said mirror including members spaced on opposite sides of said given plane, said members being adapted to provide a magnetic eld across said plane for reversing the travel of said beam and an electrostatic eld across said plane for directing said beam away from said plane, a beam target spaced outwardly from said plane and positioned so that said beam is directed onto the same, said beam target being disposed in non-parallel relation with respect to said given plane, with the spacing between said mirror and said beam target for a given cycle of beam deection in said plane varyin-g in accord with such deection to maintain focus of said beam yon said beam target.

9. An electron image display device including in combination, an elongated electron beam mirror for redirecting a beam, means supplying an electron beam and directing the same toward said mirror, deflecting means for scanning said beam in a given plane, said mirror including a plurality of electrode members spaced on opposite sides of said given plane, said electrode members being adapted to provide a magnetic field across said plane for redirecting said beam and an electrostatic eld across said plane for directing said beam outwardly from said plane, a beam target spaced outwardly from said plane and positioned so that the beam is directed 'fields in said mirror is varied therealong to provide i variable signals to said electrode members for further control of said beam.

10. An electron image display device including in combination, an elongated electron beam mirror for redirecting a beam, means supplying an electron beam and directing the same toward said mirror, deilecting means for scanning said beam in a given plane, said mirror including a plurality of electrode members spaced on opposite sides of said given plane, said electrode members being adapted to provide a time fixed magnetic field across said plane for redirecting said beam and an electrostatic eld across said plane for directing said beam outwardly from said plane, a beam target spaced outwardly from said plane and positioned so that the beam is Y directed onto the same, at least one of said electrode members being disposed in non-parallel relation with respect to said given plane so that the intensity of at least one of the fields in said mirror is varied therealong to provide uniform redirecting of the beam by said mirror with respect to said target, and means including further electrode members disposed between said mirror and said beam target to provide a time varying eld for further scanning of said beam.

11. An electron image display device including in combination, means supplying an electron beam including means for scanning said beam in a given plane, an elongated electron beam mirror for redirecting said beam, said mirror including electrode members spaced on opposite sides of said given plane and between which said beam is directed, means providing a magnetic eld between said electrode members and variable in intensity along the length of said mirror, a beam target spaced outwardly from said given plane and positioned so that the beam may be directed thereon, means for directing said beam outwardly of said given plane to impinge on said beam target, the magnetic eld intensity in said mirror being related to the transit time of said beam in said mirror and the length of the path of beam travel between said mirror and said beam target to provide a uniform trace on said beam target throughout the deflected positions of said beam in said given plane.

References Cited in the le of this patent UNITED STATES PATENTS onto the same, at least one of said electrode members 2,795,729 Gabor June 11, 1957 being disposed in non-parallel relation with respect to 2,795,731 Aiken June 11, 1957 said given plane so that the intensity of the magnetic 2,809,324 Shanafelt Oct. 8, 1957 al am 

